This invention relates to improvements in pressure domes used for coupling the blood pressure of a patient to a transducer that generates an electrical signal corresponding to the pressure. Domes used for this purpose are generally comprised of a plastic base, a wall that is usually circular having its foot adjoining the base so as to form a cavity, a flexible membrane sealed to the top of the wall so as to close the cavity, and ports extending through said base into said cavity so that the cavity can be filled with fluid.
One type of transducer with which such a pressure dome can be used is comprised of a body having a cylindrical space formed in one surface thereof and a resilient diaphragm adhered to that surface so as to close the space. An aperture may be provided that extends through the body so as to communicate ambient air pressure to the space. The bottom of the cylindrical space and the inner surface of the diaphragm are coated with conductive material so as to form plates of a capacitor having the air in the space as a dielectric. The transducer is generally contained in a pocket formed within a transducer housing. In operation, the pressure dome and the transducer housing are clamped together so that the top of the wall of the dome is in contact with the outer surface of the diaphragm around the periphery of the cylindrical space in the body of the transducer and the membrane is in intimate contact with the diaphragm. As the pressure of fluid in the cavity of the dome increases and decreases, it is applied via the flexible membrane to the resilient diaphragm so as to cause the latter to move toward and away from the bottom of the cavity in the transducer body and thereby increase and decrease the capacitance of the capacitor. Electrical connections couple the capacitor to a circuit that converts these changes in capacitance to a signal corresponding to the fluid pressure.
It has been found that in domes of the prior art variations in the temperature of the base of the pressure dome caused by changes in the temperature of the ambient air or of the fluid within the cavity of the dome cause large radial forces in the base of the wall that are translated to its top. The diaphragm of the transducer generally has a much lower thermal expansion than the plastic base and therefore does not expand and contract at the same rate as the base. As will be explained, this may cause the diaphragm of the transducer to buckle up or down so as to change the capacitance of the capacitor and cause the pressure signal to be in error. If the portion of the radial force translated to the top of the wall exceeds the force of friction between the membrane and the diaphragm in the area that is coextensive with the top of the wall, the top of the wall will slide to a different location and cause an error in the pressure signal or a zero shift. Furthermore, movement of the top of the wall with respect to the diaphragm introduces errors in subsequent pressure readings because the top of the wall does not revert to its original position on the diaphragm when the temperature of the base regains its initial value.